just take apart the center console and dash trim (only 2 pieces and about 6 screws) unbolt the old hu, figure out the wiring (if you choose not to get a harness) and think of some way to hold it in place (i.e. make some sort of mounting bracket) and ur done, pretty straight foward not sure about the coupe, but i figured out mine in about a half an hour w/o a manual.
The wiring harness makes it easy to figure out which wire goes to which one, the wires on the CD player won't "exactly" match to the ones on the current wire harness, and if you sell the car and you want to keep your CD player, all you do is unplug it rather than sit there and waste an hour trying to put the old harness back on the toyota plug.
There are very few reasons why you should forgo the harness adapter, your dad being an electrician is not one of them. It's ten bucks. If anything, your dad should be grateful that he doesn't have to do the soldering in the little hole that the radio goes in.